Conventionally, automobile power transmission systems are known in which an automatic clutch is disposed between an engine and a transmission. This automatic clutch, as disclosed for example in below Patent Citation 1 and Patent Citation 2, is configured so that a clutch mechanism for engaging/disengaging between the engine and the transmission is automatically operated by a clutch actuator.
This type of automatic clutch, for example, is configured with the same sort of parallel gear transmission as an ordinary manual transmission, and is used in combination with an automatic manual transmission (usually referred to as an AMT) that automatically performs a gearshift operation (an operation to switch gears) by employing a selection actuator, a shift actuator, and the like. Also, this automatic clutch may be combined with an ordinary manual transmission (non-AMT).
Below is a description of the overall configuration of this automatic clutch. Note that in the description below, an example is given in which a clutch disengaged state is established when using oil pressure (when applying pressure) from the clutch actuator, and a clutch engaged state is established when oil pressure is released.
The automatic clutch is provided with a friction-type clutch mechanism, and a clutch operation apparatus that operates the clutch mechanism.
The clutch mechanism, for example, is provided with a diaphragm spring for establishing the clutch engaged state by pressing a clutch disk against a flywheel attached to a crank shaft of the engine so as to revolve together with the crank shaft. Also, a release bearing is provided in order to apply/release biasing force in the axial direction on the center portion of the diaphragm spring. That is, if the release bearing is moved forward to apply biasing force in the axial direction on the center portion of the diaphragm spring, pressing force of the clutch disk against the flywheel is released, and thus the clutch disengaged state is established. Conversely, if the release bearing is moved rearward to release biasing force in the axial direction on the center portion of the diaphragm spring, the clutch disk is pressed against the flywheel, and thus the clutch engaged state is established.
Also, the above clutch operation apparatus is provided with a hydraulic control apparatus that generates an actuating force for moving the release bearing to the front and to the rear. Specifically, this hydraulic control apparatus generates oil pressure in order to operate the clutch mechanism, and by controlling the oil pressure from this hydraulic control apparatus, the automatic clutch is switched between the disengaged state and the engaged state. Also, the position (referred to below as a clutch stroke position) of the clutch disk that is set by movement of the release bearing is detected or estimated by a stroke sensor, and with feedback control based on the clutch stroke position, the aforementioned oil pressure is adjusted so that an appropriate clutch stroke position is obtained.
Note that in the description of the above automatic clutch, an example is given in which a clutch disengaged state is established when using oil pressure (when applying pressure) from the hydraulic control apparatus, and a clutch engaged state is established when oil pressure is released, but automatic clutches are also known in which conversely, a clutch engaged state is established when using oil pressure (when applying pressure) from the hydraulic control apparatus, and a clutch disengaged state is established when oil pressure is released. Below, the former automatic clutch is referred to as a “pressure-disengaged type”, and the latter is referred to as a “pressure-engaged type”.
Also, particularly in Patent Citation 2, when trouble such as a failure of the stroke sensor occurs, clutch stroke control is performed based on information such as the number of engine revolutions, thereby insuring reliability of the automatic clutch.    PTL 1: JP 2004-245325A    PTL 2: JP 2000-130474A